Tuesday, June 18, 2013

Checchucci Guitar - A Vaulted Back Body

 I have finished assembling the side ribs and vaulted back of my model of the Checchucci guitar. It was a delicate operation. The ribs and back of the original guitar are very thin, about 1 mm. Although the original instrument is intact, I was able to discern its over-all lightness and to estimate the thickness of the back and sides while examining the restoration photographs. These include a view of the vaulted back with a thumb size hole. Other views show the top edge of the side ribs with the soundboard removed.

In my last post I mentioned that I band sawed the face contour and retained the waste edges of the mould block in one piece as I was going to need them for support in cutting the longitudinal profile of the mould. They also proved useful for preserving the contour of the side ribs once they were bent. Sue and were going to be traveling in Europe for several weeks in April. I had finished contouring the vaulted back of the mould but I didn't want to start assembling the vaulted back because I knew that I couldn't finish it before we left and I didn't want to leave it half finished. But I did have time to prepare and bend the side ribs. I clamped them into the waste edges of the mould and left them until we returned home and I finished assembling the vaulted back.

For the vaulted back I prepared rib stock of macassar ebony about 1.1 mm thick. I also made  2mm wide white/black/white spacers by laminating 20 mm wide holly and ebony stock and then slicing off individual spacers. The ribs were bent to the necessary contour and shaped on an upside down jointer as seen in the top of the adjacent photo. I secured the center rib with positioning pins at each end and then applied one rib after another, alternating sides.

Each newly shaped rib was held in place with clear plastic tape and push pins. Throughout the assembly procedure the vaulting always felt fragile and as I neared completing the vault I left the plastic tape in place.

The work went quickly, in part, because Checchucci designed and constructed the vault in such a way that the shaping of each rib was simplified.

MFA Boston Restoration photo
The ribs are laid out so that the center facing edge of each rib is a straight line. I suspected this when I saw a photo of the back vault on the museum's website. When I visited the museum I arrived equipped with a straight flexible plastic ruler that I laid on the vaulted back along each rib line to test my hypothesis. Although the vaulted back was imperfectly constructed and distorted many of the ribs exhibited the center facing straight edge that I expected.

I laid out the ribs lines on my mould assuming that every rib conformed to my hypothesis. Having now assembled the vaulted back and side ribs I don't think I was completely correct.

During construction the vault always seemed fragile but when I finished gluing the vault to the side ribs I was satisfied that once I papered over the inside of the rib joints it would be stable in spite of its lightness.

I was to be disappointed. When glue dries on a papered joint it not only pulls the two sides of the joint more tightly together but it also pulls the joint down toward the inside of the instrument's body. On a arched body like a lute bowl the curvature of the ribs counteracts the downward pull and the ribs find an equilibrium.

This was not the case with the Checchucci vaulted back. The vault does not follow a uniform curve in the lower bout but flattens out over the last three ribs. I built this feature into my mould with the result that, when the glue dried on joints that I papered over, the joint between the third and fourth ribs from the edge collapsed.

The interesting point is that the original guitar suffered the same fate. In the photo it appears as a very slight deviation. If you follow the line of the joint between the third and fourth rib and the joint between the second and third ribs in the lower bout you can pick out the collapsed section just to the right of the glare.

I noticed this feature during my study of the guitar in Boston but I assumed that it was the result of distortion taking place over the long life of the guitar, not an inherit design flaw. The amount of distortion on both my guitar and the original Checchucci was only 1 mm, but it was a noticeable blemish.

Fixing the problem proved to be troublesome. I removed the three ribs that were affected by the collapsed joint and began to rebuild the area. By trial and error, in attempting to fit a new rib in place, I found that I needed to prop up the adjoining rib as seen in the photo, by about 1mm. This was possible because of the flexibility of the vault and side ribs.  The procedure not only increased the curvature of the vault slightly but also changed the shape of the new rib from one with a straight edge to one with a concave edge. This created a stronger joint that would resist the downward pull after papering over the inside of the joint. I was able to accommodate the last two ribs by filing down the side rib by a millimeter. This allowed each rib a little space to cant over, creating a slight sideways arch to an otherwise nearly flat slope.

So is this a faulty design?  I think it is a design that leaves little room for error. In instrument making there is a fine line between being in balance and being out of balance and success often depends on a finding the elusive balance.

Building replicas of historical models adds another dimension. One that is seems to be measurable, ought to be, but isn't quite.

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